Field of the Invention
The present invention relates to a sensor control apparatus which controls a gas sensor having an oxygen pump cell for pumping oxygen in or out in accordance with pump current, and to a gas detection system which includes such a gas sensor and such a sensor control apparatus.
Description of Related Art
Conventionally, there has been known a gas detection system which includes a gas sensor having an oxygen pump cell for pumping oxygen in or out in accordance with pump current, and a sensor control apparatus for controlling the gas sensor.
One example of such a gas sensor is a linear A/F sensor which performs pumping in or out of oxygen contained in a gas under measurement by using an oxygen pump cell, and detects the oxygen concentration of the gas under measurement on the basis of the pump current supplied such that the oxygen concentration of the gas under measurement becomes equal to a predetermined target concentration. Another example of such a gas sensor is an NOx sensor which detects the NOx concentration of a gas under measurement.
The sensor control apparatus must have various computation functions for controlling the pump current of such a gas sensor, and also must meet demand for reduction in the size of the apparatus. In order to satisfy these requirements, there has been used a sensor control apparatus which employs a digital control section in place of an analog circuit for performing various types of controls.
Such a sensor control apparatus including a digital control section has a digital-to-analog conversion section for converting a digital value (computation result) to an analog current. One example of the digital-to-analog conversion section is a current DAC (digital-to-analog converter) which inputs and outputs current as an analog value.
Such a digital-to-analog conversion section allows the sensor control apparatus to control the pump current (analog value) supplied to the pump cell of the gas sensor while computing the pump current by means of digital control at the digital control section.
Such a digital control section can be reduced in size as compared with an analog circuit, and can make an operation of changing control constants easier as compared with an analog circuit. Therefore, a sensor control apparatus employing such a digital control section can be easily adapted to control of gas sensors of a larger number of types and control of gas sensors having various characteristics.